1. Field of the Invention
The present invention generally relates to spinal fixation systems and the like. The present invention also generally relates to a spinal plate system that includes a mechanism for fixably attaching heads of fasteners to a spinal plate.
2. Description of the Related Art
The use of spinal fixation plates for correction of spinal deformities and for fusion of vertebrae is well known. Typically, a rigid plate is positioned to span bones or bone segments that need to be immobilized with respect to one another. Bone screws may be used to fasten the plate to the bones. Spinal plating systems are commonly used to correct problems in the lumbar and cervical portions of the spine, and are often installed posterior or anterior to the spine.
Spinal plate fixation to the cervical portion of the spine may be risky because complications during surgery may cause injury to vital organs, such as the brain stem or the spinal cord. When attaching a fixation plate to a bone, bone screws are placed either bi-cortically (i.e., entirely through the vertebrae such that a portion of the fastener extends into the spinal cord region) or uni-cortically (i.e., the fastener extends into but not through the vertebrae). Uni-cortical positioning of bone screws has grown in popularity because it is generally safer to use. Bi-cortical fasteners are intended to breach the distal cortex for maximum anchorage into the bone; however, this placement of the fasteners may place distal soft tissue structures at risk. Fastener placement is particularly important in anterior cervical plate procedures because of the presence of the spinal cord opposite the distal cortex. Unfortunately, uni-cortical fasteners may move from their desired positions because of the soft texture of the bone marrow. In fact, the portion of the bone surrounding such fasteners may fail to maintain the fasteners in their proper positions. The result is backout of the fastener.
Backout of the fastener is particularly problematic when two fasteners are implanted perpendicular to the plate. When the fasteners are placed in such a manner, backout may occur as a result of bone failure over a region that is the size of the outer diameter of the fastener threads. To overcome this problem, two fasteners may be angled in converging or diverging directions with respect to each other within the bone. The amount of bone that is required to fail before backout may occur is increased by this configuration as compared to fasteners that are implanted in parallel. Although positioning convergent or divergent fasteners in a bone reduces the risk of backout, backout may still occur.
Backout may damage internal tissue structures and cause complications if the dislocated fastener penetrates the tissue structures. For example, if backout occurs, the fastener might breach the esophageal wall of the patient. Such a breach may permit bacterial contamination of surrounding tissues, including the critical nerves in and around the spinal cord. Such a breach could be fatal.
In an attempt to reduce the risk of damage to internal tissue structures, some cervical fastener plate systems have uni-cortical fasteners that attach both to the bone and to the plate. If a fastener does backout, the fastener remains connected to the plate so that it does not contact internal tissue structures. U.S. Pat. No. 5,364,399 to Lowery et al. describes one such system and is incorporated by reference as if fully set forth herein. The Lowery et al. plating system includes a locking fastener at each end of the plate. The locking fastener engages the head of the bone screw to trap the fastener within a recess of the plate. Since the locking fastener is positioned over portions of the bone screws, the locking fastener may extend above the upper surface of the plate. Thus, the locking fastener may come into contact with internal tissue structures, such as the esophagus.
Another plating system that includes a fastener-to-plate locking mechanism is the Aline.TM. Anterior Cervical Plating System sold by Smith & Nephew Richards Inc. in Memphis, Tenn. A description of this system can be found in the Aline.TM. Anterior Cervical Plating System Surgical Technique Manual by Foley, K. T. et al., available from Smith & Nephew Richards Inc., September 1996, pp. 1-16 and is incorporated by reference as if fully set forth herein. The bone screws of this system have openings within each bone screw head for receiving a lock fastener coaxially therein. Each bone screw may be inserted into a bone such that the head of the fastener is positioned within a hole of a plate placed adjacent to the bone. The head of each bone screw is slotted so that portions of the head are deflected toward the plate during insertion of the lock fastener within the opening of the bone screw. Positioning and inserting a lock fastener within the opening can be difficult due to the small size of the lock fastener. The surgeon may be unable to hold onto the lock fastener without dropping it. If a lock fastener falls into the surgical wound, it may be difficult to retrieve. In some instances, the lock fastener may be unretrievable.